Electromagnetic translating device



May 24, 1938. J. M. KENDALL ELECTROMAGNETIC TRAN SLATING DEVJECE Filed Feb. 8. 1955 Patented May 24, 1938 UNITED STATES PATENT OFFICE James M. Kendall, Tulsa, Okla, assignor to Radio Corporation of America, a corporation of Delaware Application February 8, 1935, Serial No. 5,522

2 Claims.

My invention relates to electromagnetic translating devices and more particularly to electrical pickups of the type exemplified in the co-pending application of Emile C. Guedon, Serial No.

"5 703,742, filed December 23, 1933, and assigned to the Radio Corporation of America.

In the aforementioned co-pending application, substantially identical structure as that shown and described in the present application is dis- 110 closed. Certain features shown and described but not claimed in the co-pending application constitute the subject-matter of my invention.

In prior art devices with which I am familiar, satisfactory operating charactertistics are usually attended by a multiplicityof parts. The production cost of electrical pickups of this character is, of course, relatively high, and the time required for the assembly and adjustment of the parts thereof retards productionrate and further adds to the cost of the pickups.

It is, accordingly, the primary object of my inventionto provide an electrical pickup which, although having satisfactory operating characteristics, lends itself to cheap and speedy productlon.

A further object of my invention is to provide an electrical pickup in which the design of the parts thereof facilitates their assembly and adjustment.

A still further object of my invention is to provide in an electrical pickup improved means for adjusting and maintaining the adjustment of an armature in position in an air gap between two pole pieces thereof.

In a preferred embodiment of my invention, I employ an integral pole piece structure comprising a plate of magnetic material having two parallel spaced portions and. of substantially U-shape configuration. One of the portions of thestructure is slotted whereby two pole pieces having a non-adjustable air gap therebetween are provided for a permanent magnet. The other portion of the structure is provided with an opening opposite the air gap and a slot extending from the opening and severing an edge of the portion.

When positioned in the pickup, the portion of the pole piece structure which provides the pole pieces is positioned above and being integral therewith is supported by the other or lower portion of the structure.

Passing through the opening in the lower portion of the pole piece structure and extending into and positioned in the air gap in the upper portion is an armature which is encircled by a 'fixed pickup coil which is sandwiched in between the two portions. A strip of spring material having one end thereof fastened to the armature and the opposite end anchored in fixed relationship thereto functions to maintain the armature in position after initial adjustment in the air gap.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment when read in connection with the accompanying drawing, in which Figure 1 is a longitudinal section of an electrical pickup constructed in accordance with my invention;

Fig. 2 is a view taken on the line II--II of Fig. 1;

Fig. 3 is a view taken on the line IlI--III of Fig. 1;

Fig. 4 is a perspective view of an integral pole piece structure comprising a part of my improved electrical pickup;

Fig. 5 is a bottom view of the pole piece structure shown in Fig. 4; and

Fig. 6 is a perspective view of a frame member which is utilized to support and maintain various parts of my electrical pickup in position.

Referring to Figs. 1, 2 and 3, there is shown an electrical pickup including a casing I which serves to house the pickup unit proper. The various parts of the pickup including a permanent magnet 3 are mounted upon a frame member 5 of non-magnetic material, shown more clearly in Fig. 6, and the frame member and the pickup parts mounted thereon are retained in position within the casing l by screws 1, the heads of which are counter-sunk in the face of the casing and threaded into tapped openings 9 in the upstanding portions ll of the front of the frame member 5. It will be noted that when the frame member 5 and pickup unit are positioned within the casing l, in the manner described above, the bottom portion l3 of the frame member constitutes a cover plate for the open bottom of the casing through which the frame member with the various pickup parts mounted thereon is inserted when assembling. Also, it will be noted from an inspection of Figs. 1 and 3 that a panel [5 of insulating material which functions as a terminal board and cover plate, is located within the casing I at the rear of the pickup assembly. The panel is retained in position by a screw II which is threaded into a tapped opening 19 in an upstanding portion 2i of the rear of the frame member 5. Thus, it will be seen that the pickup unit is completely enclosed within the casing The various parts of the pickup unit which are mounted upon the frame member 5, and housed within the casing l, as described in the foregoing, include the permanent U-shape magnet 3, an integral pole piece structure 23 including pole pieces 25 for the magnet, an armature 21 and a pickup coil 29.

The integral pole piece structure 23, the design of which will be more clearly understood with reference to Figs. 4 and 5, is constituted by the substantially U-shape member 23 of magnetic material having two parallel spaced portions 24 and 26 respectively. The upper portion 24 of the member is provided with an air gap 30 which extends transversely thereof and divides the portion into the two pole pieces 25 for the respective pole faces of the U -magnet 3 which is positioned thereon. In order that the air gap 39 may be of a suitable length for the flattened end 3| of the armature 21 which is centered therein, as will be described later, the adjacent ends of the pole pieces 25 are of reduced cross sectional area as will be clearly seen in Fig. 4. Sandwiched in between the lower and upper portions of the pole piece structure 23 and encircling the armature 21 is the pickup coil 29.

The armature 21 is constituted by the flattened portion 3| which is positioned in the air gap 30, a cylindrical portion 33 extending below the flattened portion 3|, and a pivot shaft 35 extending on either side and at right angles to the cylindrical portion. The lower end of the cylindrical portion 33 is hollow to permit the insertion of a stylus 31 which is clamped in position by means of a set screw 39 which is threaded into the forward end of the pivot shaft.

Directly below the air gap 3|] in the upper portion 24 is an opening 4| in the lower portion 26 of the pole piece member 23 through which the cylindrical portion 33 of the armature 21 extends. Extending from the opening 4| and severing an edge 43 of the lower portion 26 of the pole piece member is a slot 45, which serves to prevent the said portion from acting as a short circuited turn around the opening 4|. A short circuited turn around the opening 4| would materially reduce the efficiency of the pickup. It will be seen, therefore, that the slot 45 constitutes an important feature of my invention.

A bearing for the pivot shaft 35 of the armature 21 is provided by complementary grooves 46 and in the bottom and top sides respectively of the lower portion 26 of the pole piece structure 23 and the bottom portion l3 of the frame member 5. The bearing groove 46 runs along the slot 45 from the severed edge 43 to the opening 4| and continues from the opposite side of the opening to the farther side of the pole piece structure as is clearly shown in Fig. 5. The complementary bearing groove 5| is hereinafter described in connection with the frame member 5.

The aforementioned frame member 5 on which the various parts of the pickup proper are mounted, and which is clearly illustrated in Fig. 6 of the drawing, is constituted by the bottom portion l3, the upstanding and spaced front portions connected at their tops by the portion 41, and the upstanding and spaced portions 2| and 49 at the rear of the frame member. The complementary bearing groove 5| for the armature pivot shaft 35 extends along the upper surface of the bottom portion l3 of the frame member. Means of egress for the bottom end of the armature 21 in which the stylus 31 is clamped is provided by an opening 53 located about midway the length of the groove 5|.

The complete assembly of the various parts of the pickup unit upon the frame member 5, in their cooperative relationship and their order of assembly, will now be described.

The pivot shaft 35 of the armature 21 is positioned within the bearing groove 5| in the bottom portion I3 of the frame member 5 with the bottom portion of the armature extending downwardly through the opening 53. Next, the pickup coil 29 which is in the form of a flat torus is positioned in between. the upper and lower portions of the U-shape pole piece structure 23 and the cylindrical opening 55 in the coil is aligned with the opening 4| in the lower porton 26 of the pole piece structure and the air gap 30 in the upper portion 24. The pickup coil 29 is retained in position by means of a bushing 51 of insulating material which extends below the pickup coil and fits tightly into the opening 4| in the lower portion 26 of the pole piece structure 23.

The pole piece structure 23 including the pickup coil 29 is positioned upon the bottom portion l3 of the frame member with the bearing groove 46 in the lower portion 26 of the pole piece structure enveloping the top half of the armature pivot shaft 35. When the pole piece structure 23 is so positioned, the armature 21 extends upwardly through the opening 4| and through the bushing 51, the flattened portion 3| of the armature 21 extending into the air gap 30 between the pole pieces 25. The pole piece structure 23 is retained firmly in position by screws 56 which extend through openings 58 in the bottom portion 3 of the frame member 5 and threaded into tapped openings 59 located on either side of the opening 4| in the lower portion 26 of the pole piece structure.

It will be noted in Figs. 1 and 2 that the armature pivot shaft 35 is provided with rubber bushings or sleeves 6| which are slipped thereon prior to assembly. The flexible bushings 6| function to permit movement of the armature 21 about its pivot shaft.

7 Clamped to the top portion 41 of the front of the frame member 5 by the plate and screw 63 and 65 respectively and affixed to the forward end of the pivot shaft 35 of the armature 21 is a block 61 of the inert, damping material, such as the composition commercially known as Viscoloid or the like. The damping block 61 functions to level the frequency response of the pickup.

After the several parts of the pickup unit are mounted upon the frame member 5 as described, the flattened portion 3| of the armature 21 is centered in the air gap 30 by a centering spring 69. The spring 69, which has a sharp bend about midway thereof, has an end attached to the rear end of the armature pivot shaft 35 as by extending into an opening therein and soldering, as is clearly shown in Fig. 2. The other end of the centering spring 69 is clamped between the upstanding portion 49 of the frame member 5 and a washer 1| by the screw 13 after the armature has been centered in the air gap 29. Should it be necessary to reposition the armature in the air gap, it is, of course, only necessary to release the adjusting spring by loosening the screw 13. In operation, it will be noted that the armature centering spring 69 acts in bending rather than in torsion, and it has been found that the spring functions efficiently in this manner and lends itself to easy adjustment.

After the adjustment of the armature 21 is made, the magnet 3 is placed in position with its respective pole faces in contact with the pole pieces 25, as clearly shown in Fig. 2. The assembly is then inserted through the bottom of the casing I and fastened therein. by the screws 7, as previously described. From an inspection of Fig. 2, it will be seen that the magnet 3 is clamped between the pole pieces 25 and the inwardly protruding portions 15 at the top of the casing I. Also, in Figs. 1 and 3 it will be seen that leads 11 for the pickup coil 29 are soldered to suitable clips on the terminal board I5.

The integral pole piece structure 23 which is an important part of my invention is preferably constructed in the following manner. A plate of magnetic material is bent substantially in the shape of a U to provide the two spaced parallel portions 24 and 26. Two semi-circular cuts are then made in the opposite sides of the portion 24 whereby to reduce the width of the portion substantially midway its length. A slot is then. cut transversely of the reduced portion whereby the air gap 30 and the pole pieces 25 are formed. Next, the opposite portion 26 of the structure 23 is drilled opposite the air gap 30 to form the opening 4| for the armature 21, and from the opening tothe edge 43 of the structure the slot 45' is cut toprevent the short-circuited turn as heretofore described. The groove 46 for the armature pivot shaft 35 is then cut on the under side of the lower portion 26 of the structure and the two openings 59 for fastening the structure to the frame member are tapped on opposite sides of the opening 4|.

From the foregoing, it will be seen that I have provided an electrical pickup, the various parts of 40 which are so designed that they lend themselves to cheap and speedy production and facilitate the assembly and adjustment of the pickup.

Although I have shown and described a preferred embodiment of my invention, I am fully aware that various modifications thereof may be made. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and the spirit of the appended claims.

I claim as my invention:

1. In an electrical pickup, a frame member, a permanent magnet, pol-e pieces therefor carried by said frame member and having an air gap therebetween, an armature having a shaft positioned with its axis parallel with said air gap, said armature having a portion extending into said air gap, a translating coil for said armature and means comprising a strip of spring material having one end fixed to one end of said shaft and its other end adjustably secured to said frame member, said strip extending approximately at right angles to said axis for centering said portion in said air gap.

2. In an electrical pickup, a frame member, a permanent magnet, pole pieces therefor carried by said frame member and having an air gap therebetween, an armature having a shaft positioned with its axis parallel with said air gap, said armature having a portion thereof extending into said air gap, a translating coil for said armature and means for centering said portion in said air gap, said means comprising a strip of spring material having a portion thereof bent at an angle with respect to the remaining portion and having one of said portions fastened to said armature shaft and the other portion adjustably secured JAMES M. KENDALL. 

